The present invention relates to the field of secondary radar receivers.
It is known that a radar can be fitted out with a device called a secondary radar used to obtain, from cooperating carrier vehicles equipped with radar responders, coded information elements on the identity of the carrier and other coded information elements (pertaining to altitude, reporting of radio malfunctions, distress signals etc).
The transponders of carrier vehicles emit responses whenever they are interrogated and, sometimes, they do so spontaneously in a mode of operation with selective addressing, called S mode operation, that can be used also for anticollision functions. Each radar fitted out with a secondary radar must therefore be provided with means enabling it to recognize those responses, among all the responses received, that are responses to its own interrogations, and having detected them, must decipher and validate the code of the response.
The international standardization of responses in S mode is given here below, to facilitate the understanding of the invention, with reference to FIGS. 1 and 2.
As standardized by the International Civil Aviation Organization (ICAO), a response in S mode is constituted by a train of pulses emitted on a carrier frequency of 1090 MHz.
Each train of pulses has a preamble and a message.
The preamble has four identical pulses with a duration of 0.5 .mu.s each. The first two pulses and the last two pulses are separated from each other by 0.5 .mu.s. The first pulse and the third pulse are separated from each other by 3.5 .mu.s.
The message or data block may be short or long. When it is short, the message has 56 pulses of 0.5 .mu.s each, and when it is long, it has 112. The modulation of the message is done by the position of the pulses which may be at the start or the end of 1 .mu.s intervals.
The first of these intervals is 8 .mu.s behind the first pulse of the preamble.
The definition of the standard is shown schematically in FIG. 1. This figure also shows the tolerances as defined by the ICAO.
This figure has three parts: FIG. 1a, FIG. 1b and FIG. 1c. FIG. 1a shows the general shape of an S mode response. It can be subdivided into a same preamble of 8 .mu.s plus or minus 0.05 .mu.s and a data block which may be either a short data block of 56 .mu.s or a long data block of 112 .mu.s.
FIG. 1b shows firstly the preamble and secondly the data bits of the response block. The preamble has four pulses, two first pulses and two second pulses. The two first pulses have a width of 0.5 .mu.s.+-.0.05 .mu.s and they have a spacing, between them, of 0.5 .mu.s. At 3.5 .mu.s.+-.0.05 .mu.s from the first pulse of the first block of the two pulses, there is a second block of two pulses identical to the previous one. At 8 .mu.s from the leading edge of the first pulse of the first block, come the data bits of the data block. The number of these bits is 56 or 112, depending on whether the response is short or long.
FIG. 1c shows the mode of encoding of the data bits belonging to the response block. These data bits are constituted by pulses that are each located in an interval of 1 .mu.s. Each pulse has a width of 0.5 .mu.s. It is placed either at the head of the 1 .mu.s interval or the end of this interval. If it is placed at the head, the pulse conventionally represents a one. If not it represents a zero. Thus, a pulse representing a zero followed by a pulse representing a one will be constituted by single 1 .mu.s pulse while a pulse representing a one followed by a pulse representing a zero will mean that there is no signal for 1 .mu.s.
A secondary response is formed by a pulse train. Each pulse represented in FIG. 2a has a leading edge such that, in 50 nanoseconds, a power level representing 90% of the maximum level is reached. This pulse comprises a plateau corresponding to the power level and a decreasing edge. The time difference between the point of the leading edge and the point of the trailing edge having a power level equal to 50% of the maximum power of the pulse is 0.45 .mu.s.+-.0.1 .mu.s. The pulse train shown in FIG. 2b is framed by a first pulse called F.sub.1 and a last pulse called F.sub.2. These framing pulses are always present. At time intervals that are multiples of 1.45 .mu.s, behind F.sub.1, thirteen pulses are present or not present. The presence or absence of these pulses enables the transmission of an identity or an altitude. The conventional names of the different pulses are shown in FIG. 2b. In certain cases, the pulse train has an additional pulse called SPI located at a distance of 4.35 .mu.s behind F.sub.2, namely 24.65 .mu.s.+-.0.2 .mu.s from F.sub.1.
The presence or absence of a secondary radar response is detected by the presence or absence of the pair of framing pulses F.sub.1 and F.sub.2.
Each response therefore occupies a period of 20.3 .mu.s and an additional period of 4.35 .mu.s may be needed for the transmission of the special position pulse SPI. At the speed of propagation of the waves, the total period of 24.65 .mu.s corresponds to a distance of about two nautical miles. If two aircraft are simultaneously in the major lobe of the antenna at radial distances of less than two miles, at least a part of the pulses of the messages sent by each of the aircraft will arrive in the same time interval. The use of a so-called sum channel and a so-called difference channel makes it possible to create a signal, conventionally called .DELTA./.SIGMA., representing the angular divergence of a signal received with respect to the direction of the axis of the major lobe of the antenna of the secondary receiver. The use of this signal makes it possible, to a certain extent, to separate the responses of two aircraft that are sufficiently distant from each other azimuthally. It is however frequently the case that aircraft flying at different altitudes have differences in radial distance, with respect to the receiver, of less than two miles. The partial coincidence of the responses in time may then distort the decoding of the message sent. There are two cases of mixing to be distinguished. These cases are shown in FIG. 3.
FIGS. 3a and 3b show a first train of pulses coming from a transponder A and a second train of pulses coming from a transponder B. The apparent pulse train received by the receiver is formed by pulses from A and B.
In FIG. 3a, the pulses coming from A and B are separable. There is a garbling of responses but not of pulses. In this case, the known devices can be used to separate the two responses and reconstitute the distances and the codes accurately.
In FIG. 3b, the pulses coming from A and B are not separable, at least in the case of some of them, for the end of a pulse, for example from A occurs while a pulse coming from B has started but not ended. There is a garbling of pulses. In this case, the two responses are no longer separable, and in the known systems, the corresponding responses of A and B are disturbed (false code) or one of them is not detected.
In a standard secondary radar receiver, a device known as an extractor uses the signals received by the antennas and transmitted on the sum and difference channels to create video-analog signals and video-digital signals. The analog signals are known by the names of Log.SIGMA. and .DELTA./.SIGMA.. The signal .DELTA./.SIGMA., as explained further above, represents an angular divergence between the origin of the response and the radioelectrical axis of the antenna. The signal Log.SIGMA. is a magnitude proportional to the logarithm of the power of the received signal.
The video-digital signals are known by the names Q.SIGMA. and QRSLS (quantized receiver side lobe suppression). The signals QRSLS indicates that a signal has come from a direction corresponding to a side lobe or minor lobe of the antenna. It therefore enables the elimination of such signals. The signal Q.SIGMA. is a digital signal that can takes the values 0 or 1. For this signal to have the value 1, it is necessary first of all that there should exist a signal detected by the receiver, i.e. above the detection threshold, and that furthermore the value of this signal should be less than six decibels below the maximum value of the signal. As explained further above, the duration of a pulse is measured at the level of half its power, i.e. six decibels below the maximum level. The duration for which the signal Q.SIGMA. has the value 1 therefore normally corresponds to the duration of a pulse. In the known receivers, the duration of the pulses is measured by the time during which the signal Q.SIGMA. has the value 1. In the event of a "garbling" of pulses, the "garbling" is detected by the fact that the apparent duration of a pulse is greater or not greater than a certain threshold.
It will be understood, under these conditions, that the detection of a "garbling" of pulses can be done only if the garbled pulse has a length greater than the duration of the pulse at the maximum tolerance increased by the jitter introduced into the processing devices. It follows therefrom that the known devices have a detection gap for the garbled pulses, the apparent length of which does not exceed the duration thus counted.
It must be noted that, in addition to this information gap, all that the presently used devices do, when they detect the garbling information, is to destroy the garbled information: this corresponds to a loss of information.
The present invention is aimed at improving the probability of detection of the secondary responses, not only in the case of garbling but also in the case of a response with a low signal-to-noise ratio. It is also aimed at improving the validity of the codes detected.
To this end, the invention consists of a digital processing of all the signals presently available at the level of a secondary receiver.
In an improved version, an additional signal called a frequency.SIGMA. signal is introduced, this signal being obtained from a frequency analysis of the received signal. An analysis of this kind is possible although all the transponders are supposed to emit at the frequency of 1090 MHz for, firstly, there is a tolerance as regards the frequency and, secondly, experience shows that a non-negligible number of transponders make transmission beyond the zone of the tolerances fixed by the ICAO. The frequency.SIGMA. signal is a signal representing the frequency of the received signal.